Tri-Block Copolymer and Electrolyte Membrane Made from the Same

ABSTRACT

Provided are a tri-block copolymer and an electrolyte membrane prepared therefrom. The tri-block copolymer has a structure of polar moiety-containing copolymer block/non-polar moiety-containing copolymer block/polar moiety-containing copolymer block, or non-polar moiety-containing copolymer block/polar moiety-containing copolymer block/non-polar moiety-containing copolymer block, and is useful for an electrolyte membrane for fuel cells. The electrolyte membrane for fuel cells prepared from the tri-block copolymer exhibits superior dimensional stability and excellent fuel cell performance.

TECHNICAL FIELD

The present invention relates to a tri-block copolymer and anelectrolyte membrane prepared therefrom. More specifically, the presentinvention relates to a tri-block copolymer, an electrolyte membraneprepared therefrom and a fuel cell, to which the electrolyte membrane isapplied, which may be used for an electrolyte membrane of fuel cellsuseful for distributed generation plants, cogeneration plants,pollution-free vehicle power, power for business, household power andmobile equipment powers and the like.

BACKGROUND ART

Recently, the predicted exhaustion of conventional energy sources suchas oil and coal has brought about an increasing interest in alternativeenergy sources. In particular, a fuel cell, as an energy storage system,is advantageous in that it is highly efficient, does not dischargepollutants such as NO_(x) and SO_(x), and the fuel used is abundant, andthus attracts much attention.

A fuel cell is a power generation system which converts chemical bondenergy of a fuel and an oxidizing agent into electric energy. Typically,hydrogen, methanol or hydrocarbons such as butane are used as the fueland oxygen is used as the oxidizing agent.

The most basic unit to generate electricity in the fuel cell is amembrane electrode assembly (MEA), which is composed of an electrolytemembrane, and an anode electrode and a cathode electrode formed on bothsurfaces of the electrolyte membrane. Referring to Reaction Scheme Iillustrating a mechanism via which a fuel cell generates electricity(reaction scheme of the fuel cell in the case where hydrogen is used asthe fuel), in the anode electrode, oxidation occurs to produce hydrogenions and electrons and the hydrogen ions move through the electrolytemembrane to the cathode electrode. In the cathode electrode, oxygen(oxidizing agent), the hydrogen ions transferred through the electrolytemembrane react with electrons to produce water. Based on thesereactions, electron transfer occurs in an external circuit.

At anode electrode: H₂->2H⁺+2e⁻

At cathode electrode: 1/2O₂+2H⁺+2e⁻->H₂O

Overall reaction: H₂+1/2O₂->H₂O   Reaction Scheme I

Among fuel cells, a proton exchange membrane fuel cell (also referred toas a “polymer electrode membrane fuel cell”, PEMFC) provides high energyefficiency, high current density and power density, short driving periodand rapid response to load variation. The proton exchange membrane fuelcell utilizes a proton exchange membrane and requires high protonconductivity, chemical stability, thermal stability at an operatingtemperature, low gas permeability, and in particular, superiormechanical strength as a membrane. Although membranes satisfying theserequirements have been developed, clean manufacturing technology for theproduction of price competitive membranes is needed to makecommercialization possible. Fluorine-based membranes such as Nafion(manufactured by Du Pont), Aciplex (manufactured by Dow membrane orAsahi Chemical) have disadvantages of decreased proton conductivity andhigh production cost in a low-humidity and high-temperature process.Accordingly, a great deal of research associated with non-fluorinepolymers in which a polar group is introduced into a heat-resistantpolymer as a base skeleton to provide functionalities of polymerelectrolytes is actively made. Of these, poly(arylene ether) polymershaving aromatic derivatives and ether bonds exhibit good heat resistanceand chemical resistance, superior mechanical strength, excellentdurability and low production costs.

However, dimensional stability, in consideration of the fact thatpolymer electrolyte membranes of fuel cells generate large amount ofwater, is a very important factor to be contemplated. Commonly, anelectrolyte membrane should have a high ion exchange capacity (IEC) inorder to have a high ionic conductivity. However, since ion exchangecapacity (IEC) of the electrolyte membrane is directly related to wateruptake, as water uptake increases, ion exchange capacity increases. As aresult, dimension stability is deteriorated and film thicknessincreases, thus disadvantageously deteriorating overall performance ofcells.

However, non-fluorine polymer membranes which efficiently solve theproblem of dimensional stability, while maintaining superiorperformance, have been not yet known.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention developed, as described below, a tri-blockcopolymer which has a structure in which copolymer blocks containing apolar moiety are alternately linked to copolymer blocks containing anon-polar moiety and has a weight average molecular weight of 100,000 to1,000,000 g/mol, and discovered that superior cell performance can bemaintained and dimensional stability can be improved in the case ofusing the tri-block copolymer.

The present invention was completed based on this discovery.

Technical Solution

In accordance with one aspect of the present invention, provided is atri-block copolymer as a polymer electrolyte membrane for fuel cells,which has a molecular structure of t-P-N-P-t or t-N-P-N-t, in which trepresents an end of the polymer formed by an end-forming monomer, prepresents a copolymer block having a polar moiety, and N represents acopolymer block having a non-polar moiety and the tri-block copolymer aweight average molecular weight of 100,000 to 1,000,000 g/mol.

The tri-block copolymer of the present invention has apolar-nonpolar-polar or nonpolar-polar-nonpolar copolymer blockalignment.

The tri-block copolymer is composed of three blocks, since it isprepared from an end-forming monomer. In the case where an end-formingmonomer is not used, a multi-block copolymer in which a plurality ofblocks containing a polar moiety are alternately linked to a pluralityof blocks containing a non-polar moiety is formed. It was found thatthis multi-block copolymer cannot exert the desired physical propertiesof the present invention.

The tri-block copolymer of the present invention may have a variety ofmolecular weights, depending on the specific kind of monomers used andpolymerization conditions. The weight average molecular weight oftri-block copolymer is preferably 100,000 to 1,000,000 g/mol. When theweight average molecular weight is less than 100,000 g/mol, it isdifficult to form a film or, although possible, the mechanicalproperties of the film may be poor. When the weight average molecularweight exceeds 1,000,000 g/mol, it is difficult to disperse the polymerin a solvent and processiblity may be deteriorated.

The tri-block copolymer of the present invention has a structure oft-P-N-P-t or t-N-P-N-t. The copolymer block containing a polar moiety(P) is hydrophilic, thus contributing to ionic conductivity, and thecopolymer block containing a non-polar moiety (N) is hydrophobic andthus enhances mechanical properties. Accordingly, those skilled in theart will suitably select one of the two types of tri-block copolymersfor electrolyte membranes, depending on the specific application andoperating environment.

In a preferred embodiment, P represents a copolymer block represented byFormula 1 below and N represents a copolymer block represented byFormula 2 below:

wherein A, C and V are each independently at least one selected from thegroup consisting of:

R is —NO₂ or —CF₃;

U is at least one selected from the group consisting of:

X is —SO₃H, —SO₃ ⁻M⁺, —COOH, —COO⁻M⁺ or —PO₃ ²⁻2M⁺;

M is Na or K;

B is at least one selected from the group consisting of:

W is the same as defined in U in Equation 1 and V in Equation 2;

x:y is 1000:1 to 5:1 and a:b is 1000:1 to 5:1; and

10≦m≦2000 and 5≦n≦800.

The tri-block copolymer of the present invention has lower ion exchangecapacity than a multi-block copolymer and thus exhibits superiordimensional stability. On the other hand, contrary to the assumptionthat performance of cells will be deteriorated due to the low ionexchange capacity, performance of cells can be maintained, as can beseen from the following examples. The reason for this phenomenon,although not clear, is that, although copolymers are composed of blocksprepared from the same monomer, the morphology of polymers prepared mayvary depending on the shape of the block and that the morphology of thetri-block copolymer of the present invention is different from that ofthe multi-block copolymer.

B has a side chain as well as a main chain. The content of the sidechain with respect to the total amount of tri-block copolymer may bedetermined, based upon a molar ratio of y and b.

The inventors of the present invention found that physical properties ofa copolymer varying considerably depending upon the content of sidechain. Accordingly, the tri-block copolymer of the present inventionpreferably has a specific ratio of x:y and a:b. Specifically, when themolar ratio of y to x is less than 0.001, the side chain has almost noeffect upon the polymer as a whole, and when the ratio exceeds 0.2,dispersibility of polymer dispersed in a solvent is considerablydeteriorated.

For the tri-block copolymer of the present invention, the end-formingmonomer on the end (t) of the polymer refers to a monomer which has onefunctional group which can react with other monomers (or polymerchains). Accordingly, in the polymerization process, growth of polymerchains is ceased in a region in which a bond with the end-formingmonomer is formed.

For example, the end-forming monomer is at least one selected from thegroup consisting of p-cresol, phenoxy, benzophenone and methoxy groups.

The weight average molecular weight of the trio-block copolymer mayrepresent a total of the molecular weight of the non-polarmoiety-containing block and the molecular weight of the polarmoiety-containing block and may be determined by m, n, x, y, a and b.Accordingly, the weight average molecular weight of the trio-blockcopolymer may be chosen depending on the specific application ofpolymers. For example, in the case where the trio-block copolymer isused as an electrolyte membrane for fuel cells, the tri-block copolymermay be prepared by varying m, n, x, y, a and b depending on the desiredionic exchange capacity of the copolymer.

In a preferred embodiment, the weight average molecular weight of thepolar moiety-containing copolymer block may be 3,500 to 350,000 g/moland the weight average molecular weight of the nonpolarmoiety-containing copolymer block may be 3,000 to 400,000 g/mol.

When the molecular weight of polar moiety-containing copolymer block isless than 3,500 g/mol, mechanical strength may be deteriorated duringfilm manufacture. When the molecular weight of polar moiety-containingcopolymer block exceeds 400,000 g/mol, dispersibility of copolymer in asolvent may be deteriorated and processiblity may thus be degraded.

The tri-block copolymer of the present invention may be prepared bypreparing a polar moiety-containing copolymer block having anend-forming monomer, and then preparing a non-polar moiety-containingcopolymer block and, at the same time, reacting the non-polarmoiety-containing copolymer block with the polar moiety-containingcopolymer block to prepare a tri-block copolymer having a structure oft-P-N-P-t. Alternatively, the tri-block copolymer may be prepared bypreparing a nonpolar moiety-containing copolymer block having anend-forming monomer, and then preparing a polar moiety-containingcopolymer block and, at the same time, reacting the polarmoiety-containing copolymer block with the non-polar moiety-containingcopolymer block to prepare a tri-block copolymer having a structure oft-N-P-N-t.

In one embodiment, a method for preparing the trio-block copolymer ofthe present invention in the form of t-P-N-P-t will be described below.

First, a bisphenol monomer or an aromatic dihalogen monomer; a phenolmonomer having an acid-substituent or a bisphenol monomer having anacid-substituent; or an aromatic dihalogen monomer having anacid-substituent, an end-forming monomer and a monomer having a sidechain were dissolved in an organic solvent and polymerized in thepresence of a catalyst to prepare a polar moiety-containing copolymerblock containing an end-forming monomer (S1).

In this step, when a small amount of end-forming monomer with respect tothe amount of monomer for forming the copolymer block is added, apolymer block bonded to the end-forming monomer on one end thereof canbe obtained. Since, in the process of preparing the polarmoiety-containing copolymer block in step (S1), the end-forming monomer(t) is added, a t-P type of polar moiety-containing copolymer block inwhich the end-forming monomer is linked to one end thereof can beobtained.

Any amount of the end-forming monomer may be added so long as polymerblock growth is no inhibited. For example, the content of end-formingmonomer may be 0.005 to 0.1 moles with respect to one mole of onemonomer selected from a bisphenol monomer or aromatic dihalogen monomeradded in the same process; and a phenol monomer having an acidsubstituent, a bisphenol monomer having an acid substituent or anaromatic dihalogen monomer, but is not limited thereto.

Examples of useful bisphenol monomers include, but are not limited to,4,4-biphenol, 9,9-bis(4-hydroxyphenyl)fluorene) and combinationsthereof.

Examples of useful aromatic dihalogen monomers include, but are notlimited to, 4,4′-difluorobenzophenone, bis(4-fluorophenyl)sulfone,2,2-bis(4-hydroxyphenyl)hexafluoropropane and combinations thereof.

The phenol monomer having an acid substituent, bisphenol monomer havingan acid substituent and aromatic dihalogen monomer are a phenol monomerin which at least one acid substituent is present in a phenyl ring, abisphenol monomer in which at least one acid substituent is present in aphenyl ring and an aromatic dihalogen monomer in which at least one acidsubstituent is present in a phenyl ring, respectively. Examples of thesemonomers include, but are not limited to, hydroquinonesulfonic acidpotassium salts, 2,7-dihydronaphthalene-3,6-disulfonic acid disodiumsalts, 1,7-dihydroxynaphthalene-3-sulfonic acid monosodium salts,potassium 5,5′-carbonylbis(2-fluorobenzene sulfonate)), potassium2,2′-[9,9-bis(4-hydroxyphenyl)fluorene]sulfonate and combinationsthereof. Of these, 5,5′-carbonylbis(2-fluorobenzene sulfonate)) may beprepared by directly sulfonating 4,4′-difluorobenzophenone and4,4′-difluorodiphenyl sulfone with fuming sulfuric acid, and potassium2,2′-[9,9-bis(4-hydroxyphenyl)fluorene]sulfonate may be prepared bydirectly sulfonating 9,9-bis(4-hydroxyphenyl)fluorene with ClSO₃H.

The monomer containing a side chain that can be used in the presentinvention directly constitutes a main chain of the tri-block copolymerand examples thereof include, but are not limited to,[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone, 3,5-difluoro-4′-fluorobenzophenone, (3, 5-difluoro-4′-fluorophenyl)sulfoneand combinations thereof. Of these,[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) may beprepared by Friedel-Crafts reaction of1,3,5-benzenetricarbonyltrichloride, aluminum chloride andfluorobenzene. The monomer containing other chains may also be preparedby a similar Friedel-Crafts reaction.

The organic solvent that can be used for the polymerization of the polarmoiety-containing copolymer block and/or non-polar moiety-containingcopolymer block is not particular limited so long as it is capable ofeasily dissolving reactants and products. In particular, examples of theorganic solvent include N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide(DMF) and combinations thereof.

Any catalyst generally used in the art may be used in monomerpolymerization to obtain the polar moiety-containing copolymer blockand/or non-polar moiety-containing copolymer block. A carbonate catalystis commonly used, but the present invention is not limited thereto.Examples of carbonate catalysts include, but are not limited to, sodiumcarbonate, potassium carbonate, rubidium carbonate, magnesium carbonate,calcium carbonate, barium carbonate and combinations thereof.

As described above, when respective components to prepare the polarmoiety-containing copolymer block are prepared, monomers and catalystsare dissolved in an organic solvent, stirred at 140 to 150° C. for 3 to5 hours and water is removed from the mixture. Water may be removed inthe form of an azeotrope. In this case, water may form an azeotrope withbenzene or toluene. A common Dean-Stark trap may be used for removingthe azeotrope. After the azeotrope is removed, the reaction temperatureis elevated and stirring is performed at 170 to 190° C. for 6 to 24hours as described above, to perform polymerization and thereby obtain apolar moiety-containing copolymer block having an end-forming monomer.

Then, the polar moiety-containing copolymer block having an end-formingmonomer prepared in step (S1), a bisphenol monomer, an aromaticdihalogen monomer, and a monomer having a side chain were dissolved inan organic solvent, polymerized in the presence of a catalyst to preparea non-polar moiety-containing copolymer block and, at the same time,prepare a tri-block copolymer (S2).

Both ends of the non-polar copolymer block may undergo coupling reactionwith one end of the polar moiety-containing copolymer block having anend-forming monomer in which the end-forming monomer is not linked. Thecoupling reaction enables preparation of the final tri-block copolymer(t-P-N-P-t) in which the polar moiety copolymer block (P) is chemicallylinked to both ends of non-polar moiety copolymer block (N). Asdescribed above, coupling reaction of the polar moiety-containingcopolymer block with the non-polar moiety-containing copolymer block inorder to obtain the tri-block copolymer of the present invention may becarried out simultaneously with preparation of the non-polarmoiety-containing copolymer block.

When the monomers for forming the non-polar moiety-containing copolymerblock are prepared, the non-polar moiety-containing copolymer block andthe tri-block copolymer of the present invention may be prepared inaccordance with the same reaction conditions and processes as the methodfor preparing the polar moiety-containing copolymer block. For example,after polymerization of the polar moiety-containing copolymer block, theresulting product is diluted, filtered and washed to separate the polarmoiety-containing copolymer block, a flask containing the polarmoiety-containing copolymer block is cooled to 50 to 70° C. and monomersfor the polar moiety-containing copolymer block are further addedthereto to perform polymerization.

Specifically, the monomers for forming non-polar and polarmoiety-containing copolymer blocks and catalysts are dissolved/dispersedin an organic solvent, stirred at 140 to 150° C. for 3 to 5 hours andwater is removed from the mixture. Water may be removed in the form ofan azeotrope. In this case, water may form an azeotrope with benzene ortoluene. A common Dean-Stark trap may be used for removing theazeotrope. After the azeotrope is removed, the reaction temperature iselevated and stirring is performed at 170 to 190° C. for 6 to 24 hoursas described above, to perform polymerization.

After completion of polymerization, the resulting product is directlyadded to distilled water, methanol or acetone, or diluted with distilledwater, methanol or acetone, and filtered to remove salts present in theproduct to obtain a polymer slurry. Then, the slurry was filtered,repeatedly washed with hot distilled water (˜80° C.) and/or methanol toobtain neutral pH and filtered to obtain a tri-block block copolymer.

In addition, as described above, as to a method for preparing thetri-block copolymer, preparation order of the polar moiety-containingcopolymer block and non-polar moiety-containing copolymer block may bearbitrarily selected. Accordingly, after the non-polar moiety-containingcopolymer block to which an end-forming monomer is linked is prepared,the polar moiety-containing copolymer block may be prepared. In thiscase, the tri-block copolymer in the form of t-N-P-N-t may be obtained.

The present invention also provides an electrolyte membrane comprisingthe tri-block copolymer in which N, Na or K, is substituted by H.

In Formulae 1 and 2, X represents an acid substituent. The acidsubstituent may be in the form of acid or a salt thereof depending onthe type of compound used in the preparation process. The copolymer ofthe present invention is preferably an acid in order that it can be usedfor the electrolyte membrane. Accordingly, in the case where the acidsubstituent of the tri-block copolymer is prepared in the form of a saltof an acid, the salt may be converted into an acid through addition ofan acidic solution.

That is, in the case where X is —SO³⁻M⁺, —COO⁻M⁺, —PO₃H⁻M⁺ or —PO₃²⁻2M⁺, the tri-block copolymer may be prepared in the form of anelectrolyte membrane by adding hydrochloric acid or sulfuric acid to acopolymer and converting a salt of an acid by the acid (protonation). Atthis time, preferably, the acidic solution is added to the tri-blockcopolymer in a concentration of 0.5 to 10 M and treated for 1 to 24hours. In the case where the tri-block copolymer of the presentinvention is used for an electrolyte membrane for fuel cells, sulfonateis generally substituted by sulfonic acid.

In addition, the present invention also provides a fuel cell comprising:a stack including a plurality of membrane electrode assemblies, in whichelectrodes are adhered to both ends of the electrolyte membraneaccording to claim 6, laminated such that a separator is interposedbetween the two adjacent membrane electrode assemblies; a fuel supplierto supply fuel to the stack; and an oxidizing agent supplier to supplyan oxidizing agent to the stack.

A general structure and manufacturing method of fuel cells are known inthe art and a more detailed description thereof will thus be omittedherein.

Advantageous Effects

As apparent from the fore-going, the tri-block copolymer of the presentinvention has a structure of t-P-N-P-t or t-N-P-N-t, has a low ionicexchange capacity and thus low water uptake, exhibits superiordimensional stability and maintains superior fuel cell performance, inthe case where the tri-block copolymer is used for an electrolytemembrane for fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a graph showing measurement results of performance of a unitcell as a function of an ionic exchange capacity, for electrolytemembranes of Examples 1 to 2 and Comparative Example 1 in ExperimentalExample.

BEST MODE

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1 Preparation of Polar Moiety-Containing Copolymer Block toWhich an End-Forming Monomer is Linked

1 eq. of a hydroquinonesulfonic acid potassium salt, 0.97 eq. of4,4′-difluorobenzophenone, 0.025 eq. of p-cresol and 0.02 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were added toa 1 L reactor equipped with a Dean-Stark trap and a condenser, dimethylsulfoxide (DMSO) and benzene were added as solvents and reaction wasinitiated under a nitrogen atmosphere using potassium carbonate as acatalyst. The reaction mixture was stirred in an oil bath at atemperature of 140° C. for 4 hours. The benzene was distilled, water wasremoved in the form of an azeotrope with the benzene by a molecularsieves of the Dean-Stark trap, the reaction temperature was elevated to180° C. and condensation polymerization was performed for 20 hours.

The weight average molecular weight of polar moiety-containing copolymerblock prepared was about 150,000 g/mol.

Preparation of Non-Polar Moiety-Containing Copolymer Block and Tri-BlockCopolymer

After reaction, the reaction product was cooled to 60° C., 0.34 eq. of4,4′-difluorobenzophenone, 0.335 eq. of9,9-bis(4-hydroxyphenyl)fluorene) and 0.005 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were added tothe reactor and reaction was re-initiated under a nitrogen atmosphereusing dimethyl sulfoxide (DMSO) and benzene as solvents and potassiumcarbonate as a catalyst. The reaction mixture was stirred in an oil bathat a temperature of 140° C. for 4 hours. The benzene was distilled,water was removed in the form of an azotropic mixture in the molecularsieves of the Dean-Stark trap, the reaction temperature was elevated to180° C. and condensation polymerization was performed for 20 hours.

Next, the reaction mixture was cooled to ambient temperature, DMSO wasfurther added thereto to dilute the reaction mixture and the dilutedproduct was poured into excess methanol to separate the copolymer fromsolvent. Then, the excess potassium carbonate was removed with water,the residue was filtered and the resulting copolymer was dried in avacuum oven at 80° C. for 12 hours or longer to prepare a tri-blockcopolymer in which the polar moiety-containing copolymer block ischemically bonded to both ends of the non-polar moiety-containingcopolymer block.

The weight average molecular weight of the tri-block copolymer preparedwas about 450,000 g/mol.

The tri-block copolymer prepared has a t-P-N-P-t structure and in thisexample, a molar ratio of b/a was 0.015 and a molar ratio of y/x was0.02.

Example 2

A tri-block copolymer was prepared in the same manner as in Example 1,except that 1 eq. of a hydroquinonesulfonic acid potassium salt, 0.97eq. of 4,4′-difluorobenzophenone and 0.02 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were used inthe process of preparing the polar moiety-containing copolymer block,and 0.3 eq. of 4,4′-difluorobenzophenone, 0.295 of9,9-bis(4-hydroxyphenyl)fluorene) and 0.005 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were used inthe process of preparing the non-polar moiety-containing copolymerblock.

The weight average molecular weight of polar moiety-containing copolymerblock was about 150,000 g/mol and the weight average molecular weight oftri-block copolymer prepared was about 450,000 g/mol.

The tri-block copolymer thus prepared has a t-P-N-P-t structure and inthis example, a molar ratio of b/a was 0.017 and a molar ratio of y/xwas 0.02.

Example 3 Preparation of Non-Polar Moiety-Containing Copolymer Block toWhich an End-Forming Monomer is Linked

0.97 eq. of 4,4′-difluorobenzophenone, 1 eq. of9,9-bis(4-hydroxyphenyl)fluorene), 0.02 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) and 0.025 eq.of p-cresol were added to a 1 L reactor equipped with a Dean-Stark trapand a condenser, dimethyl sulfoxide (DMSO) and benzene were added assolvents, and reaction was initiated under a nitrogen atmosphere usingpotassium carbonate as a catalyst. The reaction mixture was stirred inan oil bath at a temperature of 140° C. for 4 hours. The benzene wasdistilled, water was removed in the form of an azeotrope with thebenzene by the molecular sieves of the Dean-Stark trap, the reactiontemperature was elevated to 180° C. and condensation polymerization wasperformed for 20 hours.

The weight average molecular weight of non-polar moiety-containingcopolymer block was about 160,000 g/mol.

Preparation of Polar Moiety-Containing Copolymer Block and Tri-BlockCopolymer

After reaction, the reaction product was cooled to 60° C., 1.6 eq. of4,4′-difluorobenzophenone, 1.605 eq. of9,9-bis(4-hydroxyphenyl)fluorene) and 0.005 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were added tothe reactor and reaction was re-initiated under a nitrogen atmosphereusing dimethyl sulfoxide (DMSO) and benzene as solvents and usingpotassium carbonate as a catalyst. The reaction mixture was stirred inan oil bath at a temperature of 140° C. for 4 hours. The benzene wasdistilled off, water was removed in the form of an azeotrope with thebenzene by the molecular sieves of the Dean-Stark trap, the reactiontemperature was elevated to 180° C. and condensation polymerization wasperformed for 20 hours.

Then, the reaction mixture was cooled to ambient temperature, DMSO wasfurther added thereto to dilute the reaction mixture and the dilutedproduct was poured to excess methanol to separate a copolymer from thesolvent. Then, the excess potassium carbonate was removed with water,the residue was filtered and the resulting copolymer was dried in avacuum oven at 80° C. for 12 hours or longer to prepare a tri-blockcopolymer in which the non-polar moiety-containing copolymer block ischemically bonded to both ends of the polar moiety-containing copolymerblock.

The weight average molecular weight of tri-block copolymer prepared wasabout 480,000 g/mol.

The tri-block copolymer prepared has a t-N-P-N-t structure and in thisexample, a molar ratio of b/a was 0.02 and a molar ratio of y/x was0.0031.

Comparative Example 1

A multi-block copolymer was prepared in the same manner as in Example 1,except that 0.95 eq. of a hydroquinonesulfonic acid potassium salt, 0.97eq. of 4,4′-difluorobenzophenone and 0.02 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were used inthe process of preparing the polar moiety-containing copolymer block,and 0.23 eq. of 4,4′-difluorobenzophenone, 0.2876 of9,9-bis(4-hydroxyphenyl)fluorene) and 0.005 eq. of[3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone) were used inthe process of preparing the non-polar moiety-containing copolymerblock.

Comparative Example 2

A tri-block copolymer was prepared in the same manner as in Example 1,except that [3,5-bis(4-fluorobenzoyl)phenyl](4-fluorophenyl)methanone)as the monomer containing a side chain was not used in the processes ofpreparing the polar moiety-containing copolymer block and the non-polarmoiety-containing copolymer block; 1 eq. of the hydroquinonesulfonicacid potassium salt, 1 eq. of 4,4′-difluorobenzophenone and 0.025 eq. ofp-cresol were used in the process of preparing the polarmoiety-containing copolymer block; and 0.31 eq. of4,4′-difluorobenzophenone and 0.2975 eq. of9,9-bis(4-hydroxyphenyl)fluorene) were used in the process of preparingthe non-polar moiety-containing copolymer block.

Experimental Example Preparation of Electrolyte Membrane for Fuel Cells

The block copolymers synthesized in Examples and Comparative Exampleswere fully dissolved to a weight ratio of 3 to 15 wt % inN,N-dimethylacetamide (DMAc) as a solvent and filtered to prepare asolution for film casting. The polymer film was cast on a glasssubstrate using a doctor blade on a surface plate of applicator at aclean bench at 40° C., was allowed to stand for 24 hours, placed in anoven at 200° C. and allowed to stand for 24 hours.

Then, the glass substrate taken out of the oven was immersed in waterfor a moment and the cast polymer film was separated from the glasssubstrate. The polymer electrolyte film thus prepared was immersed in an80% sulfuric aqueous acid solution at 80° C. for 2 hours or longer toconvert potassium sulfonate of the polymer into sulfonic acid and washedwith distilled water to remove acid residue present on the surface ofthe polymer film to thereby prepare a polymer electrolyte membrane forfuel cells.

Evaluation of Water Uptake

Water uptake in accordance with an ionic exchange capacity ofelectrolyte membranes prepared in Examples 1 to 3 and ComparativeExamples 1 and 2 was measured and the results thus obtained are shown inTable 1 below:

TABLE 1 Ionic exchange capacity Water uptake Type (meq/g) (%) Ex. 1Tri-block 1.33 36.5 P-N-P Ex. 2 Tri-block 1.44 45.2 P-N-P Ex. 3Tri-block 1.12 24.2 N-P-N Comp. Ex. 1 Tri-block 1.61 57.4 — Comp. Ex. 2Tri-block — — No formation of film

As can be seen from Table 1, the tri-block copolymer of the presentinvention exhibits low water uptake and thus superior dimensionalstability, as compared to multi-block copolymers.

Evaluation of Unit Cell Performance

Unit cell performance as a function of an ionic exchange capacity ofelectrolyte membranes prepared in Examples 1 to 3 and ComparativeExamples 1 and 2 was measured and the results thus obtained are shown inFIG. 1.

As can be seen from FIG. 1, the electrolyte membranes of Examplesaccording to the present invention exhibit unit cell performancecomparable to the electrolyte membrane of Comparative Example 1.

That is, the electrolyte membrane prepared from the tri-block blockcopolymer of the present invention exhibits low water uptake and thussuperior dimensional stability, as compared to multi-block copolymers,but exhibits superior cell performance as electrolyte membranescomparable to electrolyte membranes made of multi-block copolymers.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A tri-block copolymer, as a polymer electrolyte membrane for fuelcells, having a t-P-N-P-t or t-N-P-N-t molecular structure, wherein trepresents an end of a polymer formed by an end-forming monomer, prepresents a copolymer block having a polar moiety, and N represents acopolymer block having a non-polar moiety, and wherein the tri-blockcopolymer has a weight average molecular weight of 100,000 to 1,000,000g/mol.
 2. The tri-block copolymer according to claim 1, wherein Prepresents a copolymer block represented by Formula 1 below and Nrepresents a copolymer block represented by Formula 2 below:

wherein A, C and V are each independently at least one selected from thegroup consisting of:

R is —NO₂ or —CF₃; U is at least one selected from the group consistingof:

X is —SO₃H, —SO₃ ⁻M⁺, —COOH, —COO⁻M⁺ or —PO₃ ^(2—)2M⁺; M is Na or K; Bis at least one selected from the group consisting of:

W is the same as defined in U in Equation 1 and V in Equation 2; x:y is1000:1 to 5:1, a:b is 1000: 1 to 5:1; and 10≦m≦2000 and 5≦n≦800.
 3. Thetri-block copolymer according to claim 1, wherein t is eachindependently at least one selected from the group consisting ofp-cresol, phenoxy, benzophenone and methoxy groups.
 4. The tri-blockcopolymer according to claim 1, wherein the weight average molecularweight of polar moiety-containing copolymer block is 3,500 to 350,000g/mol.
 5. The tri-block copolymer according to claim 1, wherein theweight average molecular weight of the nonpolar moiety-containingcopolymer block is 3,000 to 400,000 g/mol.
 6. An electrolyte membranecomprising the tri-block block copolymer according to claim 1 wherein N,Na or K, is substituted by H.
 7. A fuel cell comprising: a stackincluding a plurality of membrane electrode assemblies, in whichelectrodes are adhered to both ends of the electrolyte membraneaccording to claim 6, laminated such that a separator is interposedbetween the two adjacent membrane electrode assemblies; a fuel supplierto supply a fuel to the stack; and an oxidizing agent supplier to supplyan oxidizing agent to the stack.
 8. An electrolyte membrane comprisingthe tri-block block copolymer according to claim 2 wherein N, Na or K,is substituted by H.
 9. A fuel cell comprising: a stack including aplurality of membrane electrode assemblies, in which electrodes areadhered to both ends of the electrolyte membrane according to claim 8,laminated such that a separator is interposed between the two adjacentmembrane electrode assemblies; a fuel supplier to supply a fuel to thestack; and an oxidizing agent supplier to supply an oxidizing agent tothe stack.
 10. An electrolyte membrane comprising the tri-block blockcopolymer according to claim 3 wherein N, Na or K, is substituted by H.11. A fuel cell comprising: a stack including a plurality of membraneelectrode assemblies, in which electrodes are adhered to both ends ofthe electrolyte membrane according to claim 10, laminated such that aseparator is interposed between the two adjacent membrane electrodeassemblies; a fuel supplier to supply a fuel to the stack; and anoxidizing agent supplier to supply an oxidizing agent to the stack. 12.An electrolyte membrane comprising the tri-block block copolymeraccording to claim 4 wherein N, Na or K, is substituted by H.
 13. A fuelcell comprising: a stack including a plurality of membrane electrodeassemblies, in which electrodes are adhered to both ends of theelectrolyte membrane according to claim 12, laminated such that aseparator is interposed between the two adjacent membrane electrodeassemblies; a fuel supplier to supply a fuel to the stack; and anoxidizing agent supplier to supply an oxidizing agent to the stack. 14.An electrolyte membrane comprising the tri-block block copolymeraccording to claim 5 wherein N, Na or K, is substituted by H.
 15. A fuelcell comprising: a stack including a plurality of membrane electrodeassemblies, in which electrodes are adhered to both ends of theelectrolyte membrane according to claim 14, laminated such that aseparator is interposed between the two adjacent membrane electrodeassemblies; a fuel supplier to supply a fuel to the stack; and anoxidizing agent supplier to supply an oxidizing agent to the stack.